The present disclosure relates to active systems having global shutter sensor, for example a CCD sensor having a global shutter. The term “CCD” stands for Charge Coupled Device, and the pixels in a CCD store their charge until it has been depleted. A camera that has a CCD sensor almost always has also a shutter system, which can be electronic or mechanical. While this shutter system is “closed” (or “off”), the pixels can still be read because they store their charges. However, when the shutter is open, the sensor collects light, and after the shutter closes, the AFE (“Analog Front End”) reads the pixel charges one by one, dumps any excess charge, and gets the pixels ready for the next frame. In other words, the CCD captures the entire image at the same time and then reads the information after the capture is completed, rather than reading top to bottom during the exposure. Because it captures everything at once, the shutter is considered to be a “global” one. The result is an image with no motion artifacts. CCD sensors are double layered and are adapted to transfer their charges from one layer to the next before the read out is conducted.
An active system that determines depth from stereoscopic images, uses projected pattern generated by projector light source (such as a laser, a LED, a VCSEL or any similar device). In addition to the projected pattern, a measurement is conducted of observable light emitted by other sources. For example, ambient light derived from the sun or from room illumination, stray light generated as a side effect of the projector light source operation, backlight generated by a secondary controlled light source such as LED, and the like.
In order to take ambient light into consideration, it may be measured and the power of the projector/LED be modified accordingly. Alternatively, the exposure value may be set up by a user who determines an illumination profile.
The projector light source is efficient when operating under a certain range of currents, and dictates the voltage value, optical power and efficiency. Typically, the operation of a light source is efficient when the light source operates under relatively high surge current for short pulses, or when the light source operates under lower current for longer pulses, due to heating.
The projector light source may behave differently under different illumination conditions. Under intense ambient light conditions, it would make sense to generate short high power pulses, so that the projected pattern is visible at the object. On the other hand, under moderate ambient light conditions it would make sense to generate longer and weaker pulses, in order to maximize the light source efficiency as well as the available ambient light. Yet, in the absence of ambient light, it would make sense to utilize still longer pulses and a secondary light source, in order to obtain scattered light upon the object. In addition, it should be noted that the projected pattern contrast, may shift the working points.
The power provided to the projector light source needs preferably to be such that it is able to meet several requirements, among which: pulse shape and power, energy efficiency, nominal input voltage, maximal input current, low physical size and cost.
Two standard methods of providing projector power are known in the art. The first, using DC to DC converter circuits and the second, using switch capacitor voltage and current sources. In both these methods, the size and cost of the circuitry involved in order to drive effectively the projected pattern and the secondary light source, are quite limiting factors.